Well, it might be a secret about a past event that has significant implications for the present. Maybe it involves a hidden relationship or a concealed motive. But, of course, that's just speculation until you delve into the story.
Tequila originated in the area around the city of Tequila in Jalisco, Mexico. It's made from the blue agave plant. The native people in the region had been using the agave for centuries before the Spanish arrived. The Spanish influenced the production methods and later, commercial production began.
I don't have enough information about this House Castillo from the novel. However, it might be a powerful and influential house. It could have a large estate, many servants, and be involved in politics or trade. Maybe it has a unique culture within it, like special feasts or ceremonies.
There was a story of a group of friends who decided to do a tequila tasting tour in Jalisco. They visited different haciendas and tried various types of tequila. Along the way, they met an old tequila master who told them about the different flavors and how to properly taste tequila. This experience led them to discover new favorites and created memories that they would always cherish.